Corpuscular beam image detector using gas amplification by pixel type electrodes

ABSTRACT

A particle beam image detector employing gas amplification attained by pixel-type electrodes has high sensitivity and improved reliability of electrodes.  
     Electrons e −  produced through ionization of the gas by the incident particle beams drift under the force of a drift field toward a pixel that is encountered on the way to the surface of the detector, the pixel serving as an anode electrode ( 12 ). In the vicinity of the columnar anode electrode ( 12 ), by virtue of the presence of a strong electric field formed by a voltage between anode and cathode (e.g., 420 V) and the pointed shape of electrode, gas avalanche amplification of electrons occurs. The + ions thus generated quickly drift toward strip-shaped cathode electrodes ( 14 ) around the ions. In the course of this process, electric charges are generated on the columnar anode electrodes ( 12 ) and also on the strip-shaped cathodes ( 14 ), and these electric charges are observable on the electric circuit. Therefore, observation to determine the anode or cathode strip at which this amplification phenomenon occurs provides information about the position of the incident particle beam.

TECHNICAL FIELD

[0001] The present invention relates to a particle beam image detectoremploying gas amplification attained by pixel-type electrodes.

BACKGROUND ART

[0002] The present inventors have previously developed, as one type ofdetector, an MSCG (Micro Strip Gas Chamber), which is agas-amplification-type particle beam image detector realizing highposition resolution and high incident particle tolerance and containsstrip-type electrodes. Characteristic features of this detector includea very short dead time for a gas amplifier and high position resolution,and the detector has become of keen interest by virtue of its potentialuse as a detector for particle beams of high brightness. Presently,tests employing X-rays have confirmed that the detector is free ofmalfunction under a brightness of 10⁷ counts/mm² sec or more.

[0003]FIG. 1 is an exploded perspective view of a conventional MSGC.

[0004] The MSGC imaging device shown in FIG. 1 has an effective area of10 cm×10 cm. Reference numeral 1 denotes a substrate made of polyimidethin film. Reference numeral 2 denotes an anode strip formed onsubstrate 1, and reference numeral 3 denotes a strip-shaped cathodeelectrode. Anode strips 2 and strip-shaped cathode electrodes 3 arejuxtaposed alternately.

[0005] Reference numeral 4 denotes a base substrate made of ceramic, andreference 5 denotes a back-side electrode formed on the base substrate 4and placed under the substrate 1.

[0006] Approximately distance D₁ above the thus-constructed element, adrift plate 6 is provided, to thereby define a chamber for allowingpassage of gas therethrough; e.g., a gas containing argon and ethane(see, for example, Japanese patent Application Laid-Open (kokai) No.10-300856).

DISCLOSURE OF THE INVENTION

[0007] One critical problem associated with the above-described MSGCencountered during studies for putting the same into practical use isbreakage of electrodes resulting from discharge between the electrodes.In the case of the existing MSGC, a voltage is applied betweenelectrodes having a clearance of 50 μm or less. Therefore, when a highvoltage is applied in the hope of obtaining increased gas amplificationfactor, large current flows due to discharge between the electrodes. Asa result, it frequently occurs that heat generated from dischargedestroys electrode strips, or fragments of the broken electrode stripsare deposited onto the surface insulating layer, resulting inmalfunction of the device due to passage of current between theelectrodes.

[0008] Moreover, since signals generated in the back-side electrodes 5,which are two-dimensionally read out, have a magnitude about 20% that ofthe signals generated by the anodes located on the surface side, anexpensive amplifier must be employed as a circuit for attainingsuccessful readout of such weak signals, or alternatively, amplificationfactor attained by gas must be further improved.

[0009] In view of the foregoing, an object of the present invention isto provide a particle beam image detector employing gas amplificationattained by pixel-type electrodes, the detector having high sensitivityand electrodes of improved reliability.

[0010] In order to achieve the above object, the present inventionprovides the following:

[0011] [1] A particle beam image detector employing gas amplificationattained by pixel-type electrodes, characterized by comprising anodestrips formed on the back surface of a double-sided substrate, columnaranode electrodes which are planted in the anode strips such that theirupper ends penetrate the double-sided substrate so as to be exposed to asurface thereof, and strip-shaped cathode electrodes each having anaperture such that each of the corresponding columnar anode electrodesfalls therein.

[0012] [2] The particle beam image detector employing gas amplificationattained by pixel-type electrodes as recited in [1] above, wherein eachof the anode strips has a width of about 200 to 400 μm.

[0013] [3] The particle beam image detector employing gas amplificationattained by pixel-type electrodes as recited in [1] above, wherein theanode strips are provided at intervals of about 400 μm, the strip-shapedcathode electrodes each have apertures at intervals of a predetermineddistance, the diameter of the aperture being about 200 to 300 μm, andeach of the columnar anode electrodes has a diameter of about 40 to 60μm and a height of about 50 to 150 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is an exploded perspective view of a conventional MSGC;

[0015]FIG. 2 is a perspective view showing an essential portion of aparticle beam image detector employing gas amplification attained bypixel-type electrodes according to one embodiment of the presentinvention;

[0016]FIG. 3 is a plan view showing one embodiment of the particle beamimage detector employing gas amplification attained by pixel-typeelectrodes according to the present invention;

[0017]FIG. 4 is an enlarged view of the portion A indicated in FIG. 3;

[0018]FIG. 5 illustrates the operational principle of the particle beamimage detector of the present invention; and

[0019]FIG. 6 shows relations of voltage applied versus gas amplificationfactor attained by the particle beam image detector of the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0020] Hereafter, modes for carrying out the present invention will bedescribed with reference to the accompanying drawings.

[0021]FIG. 2 is a perspective view showing an essential portion of aparticle beam image detector employing gas amplification attained bypixel-type electrodes according to one embodiment of the presentinvention, FIG. 3 is a plan view thereof, and FIG. 4 is an enlarged viewof the portion A indicated in FIG. 3. In FIG. 2, for the sake of easyunderstanding of the disposition of the anode strips, the lower sectionof the double-sided printed substrate is depicted to be separated fromthe upper section. However, it should be noted that the upper and lowersections are not separated, but constitute a unitary, double-sidedsubstrate.

[0022] In these FIGs., reference numeral 1 denotes a particle beam imagedetector, 2 a pixel chamber (300 mm×300 mm), 11 an anode strip (althoughwidth d₁ is 300 μm in the present embodiment, any width falling withinthe range of about 200 μm to 400 μm may be used), 12 a columnar anodeelectrode planted in the anode strip 11 (although diameter d₂ is 50 μm,any diameter falling within the range of about 40 μm to 60 μm may beused), 13 a double-sided substrate for a printed circuit having athickness d₃ of about 100 μm; 14 a strip-shaped cathode electrode formedon one surface of the substrate 13, and 21 a drift electrode.

[0023] As shown in FIG. 2, the particle beam image detector of thepresent invention includes a double-sided printed circuit substrate 13,strip-shaped cathode electrodes 14 on one surface of the substrate 13,and anode strips 11 on the back surface of the substrate 13. The anodestrips 11 are provided at a pitch d₅ of 400 μm. The strip-shaped cathodeelectrodes 14 each have apertures 15 arranged at predeterminedintervals. At the center of each aperture 15 is provided a pixel servingas a columnar anode electrode 12. The pixel is connected to acorresponding anode strip 11 present on the back side. The diameter d₆of the aperture 15 in the strip-shaped cathode electrode 14 is 250 μm.However, the diameter is not limited thereto and may assume any valuefalling within the range of 200 μm to 300 μm.

[0024] As described above, in the present embodiment, the pixels servingas anodes 12 each have a diameter of 50 μm; however, the pixels may havea diameter of 40 μm to 60 μm. The anode electrodes 12 are of thecolumnar shape and have a height d₄ of about 100 μm, approximating thethickness of the double-sided printed substrate 13. The height of theanode electrodes is not limited to the above-mentioned specific height,and may be suitably determined within a range of 50 μm to 150 μm, inaccordance with the thickness of the double-sided printed substrate 13.

[0025] In actual use of the detector for detecting a particle beam, thedouble-sided printed substrate 13 is placed in a pixel chamber 2; i.e.,in an inert-gas-based atmosphere. As shown in FIG. 2, a drift electrode21 is provided at an appropriate position above the substrate 13 (inpractice, several mm to several cm above the substrate 13) and parallelto the detector. This arrangement allows image measurement ofradioactive rays similar to that attained by MSGC.

[0026]FIG. 5 illustrates the operational principle of the particle beamimage detector of the present invention.

[0027] Electrons e⁻ produced through ionization of the gas by theincident particle beams drift under the force of a drift field toward apixel on the substrate, the pixel serving as an anode electrode 12. Inthe vicinity of the columnar anode electrode 12, by virtue of thepresence of a strong electric field formed by a voltage between anodeand cathode (e.g., 420 V) and the pointed shape of electrode, gasavalanche amplification of electrons occurs. The + ions thus generatedquickly drift toward strip-shaped cathode electrodes 14 around the ions.

[0028] In the course of the above process, electric charges aregenerated on the columnar anode electrodes 12 and also on thestrip-shaped cathodes 14, and these electric charges are observable onthe electric circuit. Therefore, observation to determine the anode orcathode strip at which this amplification phenomenon occurs providesinformation about the position of the incident particle beam. Readingout of signals, circuit design for obtaining two-dimensional images,etc. can be performed by use of those developed for conventional MSGC.

[0029] Characteristic features of the present particle beam imagedetector are summarized as follows:

[0030] (1) Since pixels are used as anodes, strong electric fields canbe easily formed, leading to an enhancement of amplification factor.

[0031] (2) Since each cathode surrounds the corresponding anode in acircular fashion, the electric field at the peripheral portion of thecathode is much weaker than that observed at the anode. As a result,flying out of electrons from the cathode is suppressed, and thusdischarge does not easily occur.

[0032] (3) The electric field between the anode and cathode weakenssharply as a function of distance. Therefore, progress toward dischargemay occur only in rare cases.

[0033] (4) Between the anode and cathode, an insulator is provided as asubstrate. Since the width of the anode strip is larger than thediameter of the aperture of the strip-shaped cathode electrode, and thethickness of the substrate is similar to the radius of the aperture, thedirection of the line of electric force is always upward at theinsulator surface, eliminating any risk of generating the undesiredelectrostatic field caused by accumulation of positive ions generatedthrough gas amplification.

[0034] (5) Since the present particle beam image detector essentiallymakes use of techniques for fabricating printed circuit boards,detectors of large area can be produced at low cost.

[0035] (6) In the event of discharge, the detector is not fatallydamaged. That is, the only damage the detector would suffer is localbreakage (of some pixels).

[0036] (7) Since the detector operates under application of voltage toonly two terminals; i.e., an anode electrode and a drift electrode,minimum facilities in terms of power supply and wiring are required.

[0037]FIG. 6 shows relations of voltage applied versus gas amplificationfactor attained by the particle beam image detector of the presentinvention. In FIG. 6, the x-axis represents voltage (V) applied betweenthe cathode and the anode, the y-axis represents gas amplificationfactor (logarithmic scale), line “a” represents a characteristic curveaccording to the present invention, and line “b” represents thatobtained from a conventional detector.

[0038] As is apparent from FIG. 6, an amplification factor of 10,000 orthereabouts can be attained by the present invention. Also, when thedetector of the present invention was operated for two days continuouslyat an amplification factor of about 1,000, not even a single occurrenceof discharge arose. At higher amplification factors, discharge wasobserved, but rarely, with no subsequent operational problems.

[0039] Although the present invention has been described above withreference to specific embodiments, the invention is not limited to thoseembodiments. Numerous modifications and variations of the presentinvention are possible in light of the spirit of the present invention,and they are not excluded from the scope of the present invention.

[0040] As described above in detail, the present invention provides thefollowing advantages and effects among others.

[0041] (A) The detector of the invention has the same advantages asthose of MSGC. That is, the inventive detector attains a large gain andhas improved reliability of electrodes.

[0042] (B) Since pixels are used as anodes, strong electric fields canbe easily formed, leading to an enhancement of amplification factor.

[0043] (C) Since each cathode surrounds the corresponding anode in acircular fashion, the electric field at the peripheral portion of thecathode is much weaker than that observed at the anode. As a result,flying out of electrons from the cathode is suppressed, and thusdischarge does not easily occur.

[0044] (D) The electric field between the anode and cathode weakenssharply as a function of distance. Therefore, progress toward dischargemay occur only in rare cases.

[0045] (E) Between the anode and cathode, an insulator is provided as asubstrate. Since the width of the anode strip is larger than thediameter of the aperture of the cathode electrode, and the thickness ofthe substrate is similar to the radius of the aperture, the direction ofthe line of electric force is always upward at the insulator surface,eliminating any risk of generating the undesired, cancellingelectrostatic field caused by accumulation of positive ions generatedthrough gas amplification.

[0046] (F) Since the present particle beam image detector essentiallymakes use of techniques for fabricating printed circuit boards,detectors of large area can be produced at low cost.

[0047] (G) In the event of discharge, the detector is not fatallydamaged. That is, the only damage the detector would suffer is localbreakage (of some pixels).

[0048] (H) Since the detector operates under application of voltage toonly two terminals; i.e., an anode electrode and a drift electrode,minimum facilities in terms of power supply and wiring are required.

INDUSTRIAL APPLICABILITY

[0049] The particle beam image detector according to the presentinvention employing gas amplification attained by pixel-type electrodesis suitable for technical fields involving detection of radioactiverays; i.e., monitoring of radioactive rays, X-ray image analysis,medical use X-ray imaging, and new techniques of gamma-ray imaging.

1. A particle beam image detector employing gas amplification attainedby pixel-type electrodes, characterized by comprising: (a) anode stripsformed on the back surface of a double-sided substrate, (b) columnaranode electrodes which are planted in the anode strips such that theirupper ends penetrate the double-sided substrate so as to be exposed to asurface thereof, and (c) strip-shaped cathode electrodes each having anaperture such that each of the corresponding columnar anode electrodesfalls therein, the radius of the aperture is similar to the thickness ofthe said substrate, and the diameter of the aperture is smaller than thewidth of the said anode strip so that the direction of the line ofelectric force is always upward at an insulator surface, eliminating anyrisk of generating the undesired electrostatic field caused byaccumulation of positive ions generated through gas amplification. 2.The particle beam image detector employing gas amplification attained bypixel-type electrodes as recited in claim 1, wherein each of the anodestrips has a width of about 200 to 400 μm.
 3. The particle beam imagedetector employing gas amplification attained by pixel-type electrodesas recited in claim 1, wherein the anode strips are provided atintervals of about 400 μm, the strip-shaped cathode electrodes each haveapertures at intervals of a predetermined distance, the diameter of theaperture being about 200 to 300 μm, and each of the columnar anodeelectrodes has a diameter of about 40 to 60 μm and a height of about 50to 150 μm.